Your magic has become more flexible, allowing you to adapt to the current situation. If your Spellcasting feature allows you to prepare spells (eg Paladin or Wizard), once per long rest you can take a minute to swap one of your prepared spells for another spell on your spell list.
If your Spellcasting feature instead has you learn spells (eg Arcane Trickster, Ranger, or Bard), you can change one of your known spells for another spell on your spell list at the end of each long rest.